chore: ignore .feature-prompted-* gstack preamble state markers

These files are created by the gstack preamble's feature discovery section
via `touch ~/.claude/skills/gstack/.feature-prompted-*`. When the dev symlink
points .claude/skills/gstack back to the repo root, they land here instead of
in the user's home dir. They're machine-local state, never project files.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Willardgmoore

.gitignore

@@ -37,3 +37,6 @@ supabase/.temp/
 
 # Throughput analysis — local-only, regenerate via scripts/garry-output-comparison.ts
 docs/throughput-*.json
+
+# gstack preamble feature-discovery state markers (written via dev symlink)
+.feature-prompted-*